1. Technical Field
The invention relates to heavy-duty vehicles, and in particular, to frames and subframes for heavy-duty vehicles. More particularly, the present invention is directed to frames and subframes for heavy-duty vehicles which include selected components that are bolted together in a manner that provides efficient distribution of forces, and include components for absorbing energy in an extreme event during vehicle operation.
2. Background Art
Heavy-duty vehicles that transport cargo, for example, tractor-trailers or semi-trailers, and straight trucks such as dump trucks, typically include leading or trailing arm suspension assemblies that connect the axles of the vehicle to the frame of the vehicle. In some heavy-duty vehicles, the suspension assemblies are connected directly to the primary frame of the vehicle. In other heavy-duty vehicles, the primary frame of the vehicle supports a subframe, and the suspension assemblies connect directly to the subframe. For those heavy-duty vehicles that support a subframe, the subframe can be non-movable or movable, the latter being commonly referred to as a slider box, slider subframe, slider undercarriage, or secondary slider frame. For the purpose of convenience and clarity, reference herein will be made to a slider box, with the understanding that such reference is by way of example, and that the present invention applies to heavy-duty vehicle primary frames, movable subframes and non-movable subframes.
In the heavy-duty vehicle art, one or more axle/suspension systems usually are suspended from a single slider box. It is understood that a slider box outfitted with usually two axle/suspension systems typically is referred to as a slider or slider tandem, and for purposes of convenience and clarity, will hereinafter be referred to as a slider tandem. Of course, a slider box may also be outfitted with a single axle/suspension system, or three or more axle/suspension systems. By way of example, reference herein shall be made to a slider tandem having a pair of axle/suspension systems mounted thereon, with the understanding that such reference also applies to a slider outfitted with one, three or more axle/suspension systems. The slider tandem in turn is mounted on the underside of the trailer primary frame, and is movable longitudinally therealong to provide a means for variable load distribution and vehicular maneuverability.
More specifically, the amount of cargo that a trailer may carry is governed by local, state and/or national road and bridge laws, and is dependent on proper load distribution. The basic principle behind most road and bridge laws is to limit the maximum load that a vehicle may carry, as well as limit the maximum load that can be supported by individual axles. A trailer having a slider tandem gains an advantage with respect to laws governing maximum axle loads. More particularly, proper placement of the slider tandem varies individual axle loads or redistributes the trailer load so that it is within legal limits
A slider box typically includes a pair of longitudinally extending elongated main members or rails that are parallel to one another. The parallel spacing between the main members is maintained by cross members, which extend transversely between and are connected to the main members The main members and the cross members of prior art slider boxes are usually made of steel, which enables the cross members to be butted against and welded to the inboard surface of the main members. Other components that are part of or are related to the slider box, such as reinforcing members and suspension assembly hangers, typically are also made from steel and are welded to the main members and/or the cross members. The slider box typically is movably attached to the vehicle primary frame by a retractable pin mechanism.
One consideration in the design of any slider box is weight. More particularly, for at least two reasons it is desirable to reduce the weight of a slider box as much as possible, while still maintaining performance characteristics and robustness. First, such a weight reduction decreases the amount of fuel that the heavy-duty vehicle consumes, leading to a corresponding reduction in fuel costs. Second, local, state and/or national road and bridge laws typically set a maximum weight limit for a trailer load, which includes the weight of the trailer and the payload carried on or in the trailer. If the weight of the slider box is reduced, more vehicle weight capacity can be devoted to the payload, enabling a larger payload to be transported and increasing the overall profitability of the vehicle.
To reduce the weight of the slider box, the use of structural materials that are lighter than steel, such as aluminum and aluminum alloys for the main members, cross members, and other components has often been explored in the prior art. However, certain characteristics of aluminum, such as high thermal conductivity and a low melting point, make the welding of aluminum components different, and potentially more difficult, than the welding of steel components. In addition, aluminum components that are welded together may exhibit fatigue at the weld area, thereby potentially creating a weaker connection when compared to steel components that are welded together.
The potential for a weaker connection may become a concern at the interface between the main members and the hangers, and potentially at any interface between the cross members and the hangers. Since the axle/suspension system typically pivotally connects to the hangers, which are welded to the main members, the interface between the hangers and the main members is instrumental in reacting the vertical, fore-aft, side and torsional loads encountered by the axle/suspension system. More particularly, loads or forces acting on the axle/suspension system, such as brake loads, fore-aft loads, side loads, curbing loads, vertical loads and torsional loads, tend to cause the hanger to rock or move fore-to-aft and side-to-side. Such movement of the hanger highly stresses the rigid attachment of the hanger to the main member and potentially any rigid attachment of the hanger to the cross member, which may cause a potentially less-than-optimal weld to fail. Thus, the use of aluminum components, which may not facilitate a welded connection that is as strong as a weld between steel components, may undesirably fail.
To overcome the concern of a potential failure of a welded connection between aluminum components, the components may instead be bolted together. The use of a bolted connection provides strength, enables compliance, and reduces stress risers. However, in the prior art, it has been difficult to attach the hanger to the main member and/or cross member using bolts in a manner which enables adequate distribution among the bolts of the different loading forces that act on the axle/suspension system and thereby achieve a satisfactory fatigue life of the slider box components
Moreover, another consideration in the design of any slider box is the ability of the slider box to withstand extreme events. That is, the slider box and the axle/suspension system of a heavy-duty vehicle must also be durable enough to withstand the force created by events such as single-wheel impacts caused by a wheel striking a bump in a road, a large pot-hole, or highway guard rails. Such extreme events also include the static hang-up of a wheel in service, which is a low-speed event in which a tire is hung up or stopped temporarily during service until the vehicle pulls through the event. When a vehicle encounters an extreme event, a vertical crush force is produced which potentially can cause significant damage to the slider box. More specifically, in a typical prior art slider tandem, when a vertical crush force is produced, a force in the aft direction is produced wherein the beam of a trailing beam suspension pulls toward the rear of the vehicle, in turn causing the rear portion of the hanger to which it is pivotally attached to impact or move vertically upward into the main member and/or cross member with significant force.
This vertical crush force may be of differing magnitudes at different points throughout the suspension system, depending on the nature of the impact. For example, a static hang-up of a wheel in service is likely to produce a greater force than simply striking a bump in the road. A side force may also be produced if the impact is on a single wheel, which would cause the beam to pull back and sideways, causing the hanger to twist. These impacts could damage, or in an extreme case, cause a slider box main member and/or one or more of the attached cross members to fail, in either instance eventually requiring replacement, which is costly and time-consuming. Although the hanger typically is not damaged from such impacts, it usually also is replaced along with the main member and/or cross member. This design of a typical slider tandem causes many heavy-duty vehicles containing such slider tandems to be out of service for extended periods of time after such extreme events until the entire slider box can be replaced.
These potential concerns have created a need in the art for lighter weight heavy-duty vehicle primary frames and subframes that include selected components which are joined in a stronger and more dependable manner than by welding nonferrous materials, which are capable of efficient distribution of forces, and which reduce potential damage from extreme events.